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Thread: Fishman Loudbox PRO-LBX-300

  1. #36
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    Hello all, hereís what I have gotten to. I disassembled the amp, and looked for any issues from manufacturer repair on the power amp board. I didnít notice any; the work actually looks nicely done. I did notice that the components that I had removed appear to have been reused. Which is odd? I noticed this when I received the board, but didnít think anything of it. I figured I removed them in error, and they were still good, thus, reused. I checked for shorts between heat sink and tabs. All checked out fine. There is a cable between the cab and the control board that is epoxied in place, so it is semi not removable. It is the Molex connector cable that goes in between connector JP8 on the Power Amp board and preamp channel two control board. Quite a PIA to semi assemble this for diagnostic/repair. I pinned this cable out to make sure there were no shorts and there was continuity. It came back fine. I then semi assemble the amp in such a way that I can access everything, since preamp boards are stacked. I started at the mains to make sure I had power, which I obviously do. The transformer reads 27V AC on it, and that's what's coming into the power board. It goes through a Wheatstone bridge and changes from 27v ac to 35.4 and -37.6 vdc which on the schematic should be +/- 33V. I donít know what the acceptable tolerance levels for the voltages are for the voltages, +/- whatever %. On U10/U11 the VCC/VEE voltages are listed as +/- 14V. On U10 I am getting 13.1 and U11 reads -16.1. I follow this trough the power amp board up to the JP8 header. I have 13.1 at pin 2 and -16.1 at pin 4. This travels through the cable to the Preamp channel 2 board where there is 13.1 at pin two and -16.1 at pin 4. There is -16.1 running from the Preamp CH2 board to the Preamp CH1 board through JP3. I cannot find any positive voltage on the Preamp CH2 board.

    Where do I go from here?

  2. #37
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    Here's the entire schematic...
    Attached Files Attached Files
    Jazz P Bass likes this.

  3. #38
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    The 3V less on the + side seems a bit much but that is something to look at later.
    You say you have +13 at JP8 pin2, it should also be at JP3 pin1, is it?
    Check the solder connection at JP8 pin2. Ok? Follow the trace from pin2 and look for cracks or burns or blown out areas on the trace.
    Certified Dotard

  4. #39
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    Just as a side comment, I fond it WEIRD the statedn power ratings written on thge schematics.
    Like Enzo said: "the amplifier is that thingie between power supply and speakers" .... with which I fully agree.Taking circuit losses aside, the +/-V rails are the peak voltage available for the load so in an SS amp they instantly define possible power out.
    * Rails are +/-33V . Our friend gibson17 measured slightly higher values, still within spec.
    * Bass amp states 100W into 4 ohms.
    Impossible with +/-33V , just possible with +/-35/37V . Accepted.
    * Mid amp is exact same circuit, same rails, but states 40W into 4 ohm? WTF?
    It will put out exact same power as Bass amp, whatever that is, under the same load.
    * Tweeter amp is an LM3886 , is fed exact same rails (which are too much for it) , now states 20W into 4 ohms. WTF?????

    Ok, letīs see what the fresh measurements are, hope amp can be repaired.
    Juan Manuel Fahey

  5. #40
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    Nothing on JP3 pin 1. I clearly don't know much, but I figured 13.1 was closer to the 14 spec than 16.2. I don't see any blown or burnt traces, but to my untrained eye they are hard to follow. They appear to disappear into some kind of turret or test point. It might have something to do with the dual layer board? I'm going to include a few photos so you can see what I am looking at. As Enzo pointed out, I don't have the correct terminology and knowledge, which is why I am here. I'm trying my best to not sound like a doofus and communicate as clear as possible.

    I didn't specify where I was getting -16.1 on JP3. I am assuming that the connectors are running in the same direction, pin wise as they are all running in a row right next to eachother. So if that's the case, then I am getting -16.1 on pin 8. If not, then it's pin 3. Either way, by the schematic, it looks like it shouldn't be on either. On JP3, every pin is reading - voltage, with the exception of what I think is pin 1 which is running 0.00.

    Thank you all for your help, I really appreciate it.

  6. #41
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    Preamp Ch2 Board, Top.

    20180213_163741.jpg

  7. #42
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    Preamp CH2 Board, Underside

    20180213_163810.jpg

  8. #43
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    Quote Originally Posted by gibson17 View Post
    I am assuming that the connectors are running in the same direction, pin wise as they are all running in a row right next to eachother. So if that's the case, then I am getting -16.1 on pin 8. If not, then it's pin 3. Either way, by the schematic, it looks like it shouldn't be on either. On JP3, every pin is reading - voltage, with the exception of what I think is pin 1 which is running 0.00.
    I think you may be missing a ground. Some amps require everything to be assembled for grounding. Sometimes there are traces that connect to screws, or pot casings that carry the ground to chassis. But I'm just speculating.
    As far as the connectors, look at the schematic (I've attached the applicable one as the zip file was a bit difficult to navigate).
    JP8 shows 3 ground connections, pins 6,8, and 10. Now look at your photo from post #42 (underside). All the green mass is the ground plane. You can see 3 pins of the upper connector are connected to the green mass. So those are pins 6,8,& 10.
    You should be able to similarly identify the other connectors, normally we would expect them to use the same orientation, pin 1 on the left. But always check to be sure if they are not identified.
    Once you have the numbering figured out, do some resistance checks. Pin2 of JP8 should be connected to Pin1 of JP3.
    Pin2 of JP8 should also connect to pin8 of all the 33078 IC's on that ch.2 board. And pin4 of JP8 should connect to the 33078 pin4's.
    If you do not get direct connections, follow the traces. It will be tough with double sided board.
    If everything checks out, look for grounds that are missing due to the disassembly.
    Attached Files Attached Files
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